1. Field of the Invention
This invention relates to a coaxial 1-of-n relay transfer switch having reed switches (reed contacts) arranged in respective electrically conductive, non-magnetic tubes, the reed switches being arranged in a star configuration in such a way that one end of each reed switch connects with like ends of the other reed switches at a branching point within a metallic head enclosing the branching point, and are there connected to an inner conductor of a coaxial input line, and the other ends of the reed switches are connected to respective coaxial output lines.
2. Description of the Prior Art
In test devices for rapid digital switching circuits the object often exists to connect, in a program-controlled manner, one of n input terminals of a test object to the output of a generator, and to an input of an oscillograph via coaxial lines and transfer switches. The connection is to be accomplished in such a manner that, down to signal rise times of 0.5 ns, the network formed in each case represents, to the greatest possible extent, a reflection-free, wide-band 50 ohm coaxial connection of specific electrical length. The same is true for the program-control connection between one of q outputs of the test object and another input of the oscillograph.
A technical solution for the above problem is commonly found in relay matrices having one input and n or q, respectively, outputs, for example n = q = 64, which are composed of individual relay transfer switches connected in cascade, for example in the configuration 1-of-4.
FIG. 1 illustrates the basic circuit diagram of a 1-of-4 transfer switch of the type just mentioned. The coaxial input line E1 is to be connected, via one of the four relay-controlled reed switches r1 . . . r4 to one of the four coaxial output lines in such a way that, to the extent possible, a reflection-free, low distortion and low attenuation pulse transmission is possible in both transmission directions.
In order to accomplish this objective, various possibilities have become known in the art. Thus, for example, it is known to arrange the four reed switches belonging to a 1-of-4 transfer switch next to one another on a printed circuit board, to connect the reed switches on one side to one another and to connect a 50 ohm input line to this connection, while the other ends of the reed switches are directly connected to 50 ohm output lines.
Further, it has become known to arrange four reed switches in a star shape on the top side of a multi-layer printed circuit board and to enclose the branching point of the contacts on the top of the multi-layer printed circuit board within a metallic head, into which a coaxial plug connection is inserted from the bottom side of the multi-layer printed circuit board. The reed switches are connected to coaxial output plug connections in that the connecting legs of the reed switches are connected to soldering eyes, and 50 ohm strip lines extend from the soldering eyes to the coaxial output plug connections. In order to transfer, in the region of the soldering eyes, the quasi-coaxial lines, which the inner conductors form with the copper tubes enclosing the reed switches, as jolt-free as possible into the 50 ohm strip lines, the tube ends are connected to the ground plane of the printed circuit board, by way of wires and other soldering eyes.
For the following reasons, arrangements of this type cannot be produced with sufficient band width and sufficiently low reflection:
1. The junctions from the reed contacts to the 50 ohm strip lines with connection wires and a zero volt wire are affected by mechanical shock.
2. The soldering eyes disturb the lines with a capacitance of about 2 pF at about zero volts.
3. The junctions from the 50 ohm strip lines to the output sockets are affected by mechanical shock.
Therefore, fundamental reflections of &gt;20% (at 150 ps generator rise time), band widths of &lt;1 GHz and residual time constants of &gt;500 ps are to be anticipated.